1. Field of the Invention
This invention relates generally to catadioptric imaging systems, including for example offset systems for imaging documents.
2. Description of the Related Art
To achieve a high-resolution image of a paper document, current desktop document imaging systems typically either perform some form of mechanical scanning, use a large working focal distance, and/or operate on-axis (i.e., the document is centered on and perpendicular to the optical axis of the imaging system). Current desktop document imagers can generally be classified into two broad categories: traditional document scanners and “tall” document imagers.
FIG. 1 shows an example of a traditional document scanner. The document is placed on the scan surface and a scan mechanism is mechanically scanned across the document. The device relies on mechanical scanning to create a two-dimensional image using a one-dimensional image capture device. The mechanical flatbed document scanner has several disadvantages. First, the scanning process is slow and noisy. Second, the document scanner is bulky, which reduces its portability and makes it unattractive as a desktop accessory. Third, the device consumes more power due to the mechanical scanning process.
FIG. 2 shows an example of a “tall” document imager. In these systems, the document image is captured using a two-dimensional image capture device located sufficiently far away from the paper document. Such a system requires a fairly long working distance from the document, so the camera's optical axis can be both perpendicular to and centered on the document. Reducing the operating height for such a perpendicular imaging system typically requires a wide angle imaging system, which would introduce severe barrel distortions at large field angles (i.e. along the outer boundaries of the document).
Some imaging systems, such as whiteboard capture imaging systems, have an optical subsystem which is not perpendicular to the document surface, allowing a lower system height. These imaging systems, however, create undesirable keystoning of the optical image. This keystoning is unacceptably deleterious for many paper document imaging applications, as it introduces a variable image resolution over the document. The foreshortening of the image can reduce the document scanning resolution considerably.
FIG. 3 illustrate the deleterious effects of distortion. FIG. 3a visualizes a two-dimensional sensor array (grid) overlayed on a severely distorted image of a document, such as would be produced by a conventional wide-angle lens system. While the sensor may achieve the targeted resolution at the center of the image, at the periphery the image resolution is severely compromised due to the geometric distortion. FIG. 3b shows a two-dimensional sensor array (grid) overlayed on a document image with significant keystoning. In this case, the document may achieve the targeted resolution at one edge of the document, but the foreshortening reduces the resolution dramatically at the opposite edge.
Thus, there is a need for document imaging systems that overcome some or all of these drawbacks. Such systems preferably should avoid mechanical scanning and should have a relatively short working distance and small overall size.